The present invention is directed to integrated circuits. More particularly, the invention provides systems and methods for dimming control. Merely by way of example, the invention has been applied for dimming control using a light dimmer with capacitive loads. But it would be recognized that the invention has a much broader range of applicability.
Light emitting diodes (LEDs) have been widely used in various electronics applications, such as architectural lighting, automotive lighting, and backlighting of liquid crystal display (LCD). LEDs have been recognized to have significant advantages over other lighting sources, such as incandescent lamps, and the advantages include at least high efficiency and long lifetime. But, significant challenges remain for LEDs to widely replace incandescent lamps. The LED light systems need to be made compatible with conventional light dimmers that often operate with a phase-cut dimming method, such as leading edge dimming or trailing edge dimming.
Specifically, a conventional light dimmer usually includes a Triode for Alternating Current (TRIAC), and is used to drive pure resistive loads, such as incandescent lamps. But such conventional light dimmer may not function properly when connected to capacitive loads, such as LEDs and/or associated circuits. When the light dimmer starts conduction, internal inductance of the light dimmer and the capacitive loads may cause low frequency oscillation. Hence, the Alternate Current (AC) waveforms of the light dimmer often becomes unstable, resulting in flickering, undesirable audible noise, and/or even damages to other system components. FIG. 1 shows simplified signal waveforms of a conventional light dimmer that is connected to capacitive loads. The waveform 110 represents a rectified input waveform, and the waveform 120 represents a signal generated from a light dimmer.
In attempt to solve the above problems in using a conventional light dimmer with capacitive loads such as LEDs and/or associated circuits, a power resistor (e.g., with a resistance of several hundred Ohms) may be connected in series in an AC loop to dampen initial current surge when the light dimmer starts conduction.
FIG. 2 is a simplified diagram of a conventional light dimmer circuit. The light dimmer circuit 200 includes an AC input 210, a light dimmer 220, a capacitive load 230, and a power resistor 240. Additionally, FIG. 3 shows simplified conventional signal waveforms of the light dimmer circuit 200. As shown in FIGS. 2 and 3, the waveform 310 represents a rectified input signal received by the light dimmer 220. In response, the light dimmer 220 generates an output signal that is represented by the waveform 320 and received by the capacitive load 230. Comparing the waveforms of FIG. 3 with those in FIG. 1, using the resistor 240 in the light dimmer circuit 200 can reduce low frequency oscillation. But, for the light dimmer circuit 200, a current would flow through the resistor 240 even under normal working conditions, causing excessive heating of resistor and other system components. Such heating often leads to low efficiency and high energy consumption.
Therefore, some conventional techniques would short the power resistor through peripheral circuits when the AC input is stabilized after a light dimmer conducts for a predetermined period of time. FIG. 4 is a simplified conventional diagram showing a system for dimming control. As an example, a TRIAC (not shown in FIG. 4) is used as a light dimmer. The system 400 includes input terminals 422 and 424, a capacitor 430, a TRIAC dimming control circuit 440, and output terminals 452, 454. The TRIAC dimming control circuit 440 includes a power transistor 460, and resistors 472, 474, 476 and 478. As shown in FIG. 4, the TRIAC sends an input signal 410 to the input terminals 422 and 424. When the TRIAC is turned off, there is no input signal 410. In response, the transistor 460 is turned off by the voltage divider including the resistors 472, 474 and 476. When the TRIAC is turned on, the transistor 460 remains off, but the resistor 478 can dampen an initial surge current. After a predetermined period of time, the transistor 460 is turned on, and hence the resistor 478 is shorted. Therefore, the above noted approach can improve the system efficiency.
But the system 400 still suffers from significant deficiencies. For example, in a BUCK topology, when the TRIAC is turned off, the voltage on the capacitor 430 may not become lower than the output voltage (e.g., VOUT) at output terminals 452 and 454. If the output voltage and/or the threshold voltage of the transistor 460 changes, the transistor 460 may not be turned off properly and thus the resistor 478 may always be shorted. Thus, the system 400 would not operate properly under these circumstances.
Hence it is highly desirable to improve techniques of dimming control.